The conventional magnetic memory has been developed for many years, and in recent years, a theory and experiment of current-driven domain wall motion is provided and well developed. The document of U.S. Pat. No. 6,834,005 discloses a device structure capable of greatly increasing an information storage capacity of a chip or a hard disk, and such device structure is also referred to as a magnetic shift register memory. Such memory has a chance to substitute a current dynamic random access memory (DRAM), a static random access memory (SRAM) and a flash chip, and can even implement a technique of “disk drive on a chip”. Such memory mainly applies a magnetic record disk similar to a hard disk, which is folded in a three-dimensional stack for storing data, in which the current drives a domain wall motion to record information therein. The stacking effect can greatly reduce an equivalent bit size thereof, and an operation speed thereof exceeds that of a conventional flash memory chip and a hard disk.
FIG. 1 is a schematic diagram illustrating a conventional magnetic shift register memory designed according to the current-driven domain wall motion mechanism. Referring to FIG. 1, a magnetic track 100 has a plurality of magnetic domains 102. A magnetizing direction of each magnetic domain 102 can store one bit data of “0” or “1”. When the bit data of two adjacent magnetic domains are different, it represents that the magnetizing directions of the two magnetic domains are reversed, and a domain wall 104 is generated between the two magnetic domains. The domain wall 104 is shifted according to a flowing direction of an electronic current I, i.e. the magnetic domains 102 on the magnetic track 100 are shifted. When the magnetic domain 102 is shifted to pass a reading device 106 or a writing device 108, the reading device 106 and the writing device 108 can read data on the magnetic domain 102, or write data onto the magnetic domain 102.
Magnetic materials of the magnetic memory cell are classified into in-plan magnetic anisotropy (IMA) materials and perpendicular magnetic anisotropy (PMA) materials. FIG. 2 is a schematic diagram illustrating a storage mechanism of a conventional IMA material memory cell. Referring to FIG. 2, a plurality of magnetic domains on a magnetic track 120 respectively stores one bit data by the magnetizing direction. A reading device 122 identifies the magnetizing direction of the magnetic domain passed thereby, and determines whether the stored bit data is “0” or “1” according to a read magnetoresistance.
FIG. 3 is a schematic diagram illustrating an operation method of a conventional magnetic track. Referring to FIG. 3, according to a driving mechanism of FIG. 1, the magnetic track 120 is formed in a U-type structure. As the domain walls are driven by the electronic current, the domain walls sequentially pass through the reading device 122. The reading device 122 read the magnetoresistances to identify the bit data. Since the IMA material memory cell has a relatively long length, and a width of the domain wall is relatively large, which may occupy a larger area, a magnetic track made of the PMA material is preferred.
FIG. 4 is a schematic diagram illustrating a storage mechanism of a conventional PMA material memory cell. Referring to FIG. 4, a magnetic track 124 of the PMA material also includes a plurality of magnetic domains, though a magnetizing direction of each magnetic domain is perpendicular to the magnetic track 124. The reading device 122 detects the magnetizing direction of each magnetic domain to determine the stored bit data. However, a magnetic moment of the PMA material has to conquer a demagnetisation field to obtain a better perpendicular magnetic anisotropy, so that a fabrication process thereof is relatively difficult.
A data writing process is similar to the data reading process of the reading device 122, by which a magnetizing direction of the pass-by magnetic domain is changed to a desired magnetizing direction, so as to achieve the write function. The conventional writing device 108 of FIG. 1 generates a magnetic field to the magnetic domain for writing data. Regarding the writing device, an efficiency of generating the magnetic field is relatively low, which may consume relatively more power, and the writing device is not easy to be miniaturized.
How to effectively and correctly write data to the magnetic domain while considering a stability and a smaller size of the device is one of important development directions of the memory device.